Structure and bonding in monomeric iron(III) complexes with terminal oxo and hydroxo ligands

We report on the structure and bonding in the title iron(III) complexes, containing the tris[(N'-tert-butylureayl)-N-ethyl]amine ligand, with density functional theory techniques. In agreement with the experimental data, a high-spin electronic state is favored for all of the systems we considered. H bonds between the terminal oxo and hydroxo ligands and NH groups present in the organic ligand coordinated to the metal have a remarkable effect on the overall coordination geometry. In fact, the structure of model complexes without H bonds shows shorter Fe-O bond lengths. This is a consequence of the ability of the H bonds to stabilize a remarkable amount of electron density localized on the terminal oxo and hydroxo ligands. Energy analysis indicates that each H bond stabilizes the nonheme complexes by roughly 35 kJ/mol. Molecular orbital analysis indicates a reduction of two Fe-O bonding electrons on going from a complex with a terminal oxo ligand to a complex with a terminal hydroxo ligand. This reduction in the number of bonding electrons is also supported by frequency analysis.     

Utilization of hydrogen bonds to stabilize M-O(H) units: synthesis and properties of monomeric iron and manganese complexes with terminal oxo and hydroxo ligands

Non-heme iron and manganese species with terminal oxo ligands are proposed to be key intermediates in a variety of biological and synthetic systems; however, the stabilization of these types of complexes has proven difficult because of the tendency to form oxo-bridged complexes. Described herein are the design, isolation, and properties for a series of mononuclear Fe(III) and Mn(III) complexes with terminal oxo or hydroxo ligands. Isolation of the complexes was facilitated by the tripodal ligand tris[(N'-tert-butylureaylato)-N-ethyl]aminato ([H(3)1](3-)), which creates a protective hydrogen bond cavity around the M(III)-O(H) units (M(III) = Fe and Mn). The M(III)-O(H) complexes are prepared by the activation of dioxygen and deprotonation of water. In addition, the M(III)-O(H) complexes can be synthesized using oxygen atom transfer reagents such as N-oxides and hydroxylamines. The [Fe(III)H(3)1(O)](2-) complex also can be made using sulfoxides. These findings support the proposal of a high valent M(IV)-oxo species as an intermediate during dioxygen cleavage. Isotopic labeling studies show that oxo ligands in the [M(III)H(3)1(O)](2-) complexes come directly from the cleavage of dioxygen: for [Fe(III)H(3)1(O)](2-) the nu(Fe-(16)O) = 671 cm(-1), which shifts 26 cm(-1) in [Fe(III)H(3)1((18)O)](2-) (nu(Fe-(18)O) = 645 cm(-1)); a nu(Mn-(16)O) = 700 cm(-1) was observed for [Mn(III)H(3)1((16)O)](2-), which shifts to 672 cm(-1) in the Mn-(18)O isotopomer. X-ray diffraction studies show that the Fe-O distance is 1.813(3) A in [Fe(III)H(3)1(O)](2-), while a longer bond is found in [Fe(III)H(3)1(OH)](-) (Fe-O at 1.926(2) A); a similar trend was found for the Mn(III)-O(H) complexes, where a Mn-O distance of 1.771(5) A is observed for [Mn(III)H(3)1(O)](2-) and 1.873(2) A for [Mn(III)H(3)1(OH)](-). Strong intramolecular hydrogen bonds between the urea NH groups of [H(3)1](3-) and the oxo and oxygen of the hydroxo ligand are observed in all the complexes. These findings, along with density functional theory calculations, indicate that a single sigma-bond exists between the M(III) centers and the oxo ligands, and additional interactions to the oxo ligands arise from intramolecular H-bonds, which illustrates that noncovalent interactions may replace pi-bonds in stabilizing oxometal complexes.     

Structural features of monomeric octahedral d 2 rhenium(V) oxo complexes with oxygen atoms in trans-positions to oxo ligands: Complexes with hydroxo ligands in trans-positions to O(oxo)

Some structural features of mononuclear octahedral rhenium(V) oxo complexes with oxygen atoms of hydroxo ligands in trans-positions to multiply bonded oxo ligands are considered. The complexes contain monodentate inorganic or organic ligands or bi- and tetradentate organic ligands in the equatorial plane.     

Isolation of monomeric Mn(III/II)-H and Mn(III)-complexes from water: evaluation of O-H bond dissociation energies

The syntheses and properties of the monomeric [MnIII/IIH31(OH)]-/2- and [MnIIIH31(O)]2- complexes are reported, where [H31]3- is the tripodal ligand tris[(N'-tert-butylureaylato)-N-ethyl)]aminato. Isotope-labeling studies with H218O confirmed that water is the source of the terminal oxo and oxygen in the hydroxo ligand. The molecular structures of the [MnIIH31(OH)]2- and [MnIIIH31(O)]2- complexes were determined by X-ray diffraction methods and show that each complex has trigonal bipyramidal coordination geometry. The MnIII-O distance in [MnIIIH31(O)]2- is 1.771(4) A, which is lengthened to 2.059(2) A in [MnIIH31(OH)]2-. Structural studies also show that [H31]3- provides a hydrogen-bond cavity that surrounds the MnIII-O(H) units. Using a thermodynamic approach, which requires pKa and redox potentials, bond dissociation energies of 77(4) and 110(4) kcal/mol were calculated for [MnIIH31(O-H)]2- and [MnIIIH31(O-H)]-, respectively. The calculated value of 77 kcal/mol for the [MnIIH31(O-H)]2- complex is supported by the ability of [MnIIIH31(O)]2- complex to cleave C-H bonds with bond energies of <80 kcal/mol.     

Semiempirical computation of homolytic O-H bond dissociation energies of alcohols: comparison of the AM1 and PM3 methods

The heats of formation of various alcohols and alkoxy radicals were calculated using the AM1 and PM3 semiempirical methods, which were then used to calculate the bond dissociation energies of the alcohols. Both restricted Hartree-Fock (RHF) and unrestricted Hartree-Fock (UHF) calculations were performed to determine which technique was most applicable to the computation of bond dissociation energies within the semiempirical frameworks. It was determined that AM1/RHF calculations gave the most accurate results for O-H bond dissociation energies of alcohols. The effect of using configuration interaction calculations to calculate bond dissociation energies within the semiempirical framework was also examined.     

A new reactivity pattern of low-valent transition-metal hydroxo complexes: straightforward synthesis of hydrosulfido complexes via reaction with carbon disulfide

A new basic transformation linking two important classes of transition metal compounds; namely, hydroxo and hydrosulfido complexes has been discovered.     

Bond dissociation energies of ligands in square planar Pd(II) and Pt(II) complexes: An assessment using trans influence

DFT calculations using MPWB1K method with COSMO continuum solvation model have been carried out to quantify the trans influence of various X ligands (E_X) in [Pt^IICl₃X]ⁿ⁻ complexes as well as the mutual trans influence of two X and Y ligands (E_XY) in [Pt^IICl₂XY]ⁿ⁻ complexes. A quantitative structure energy relationship (QSER) is derived for predicting the E_XY using E_X and E_Y and this relationship showed a strong similarity to a QSER derived for predicting E_XY of [Pd^IICl₂XY]ⁿ⁻ complexes. Quantification of the contributions of E_X and E_XY to the bond dissociation energy of the ligand X (BDE_X) in complexes of the type [M^IIX(Y)X′(Y′)] (M = Pd, Pt) is also achieved. The BDE_X of any ligand X in these complexes can be predicted using the equations, viz. BDE_X(Pd) = 1.196E_X − 0.603E_XY − 0.118E_X’Y’ + 0.442D_X + 15.169 for Pd(II) complexes and BDE_X(Pt) = 1.420E_X − 0.741E_XY − 0.125E_X’Y’ + 0.498D_X + 13.852 for Pt(II) complexes, where D_X corresponds to the bond dissociation energy of X in [M^IICl₃X]ⁿ⁻ complexes. These expressions suggest that the mutual trans influence from X and Y is more dominant than the mutual trans influence from X′ and Y′ and both factors contribute significantly to the weakening of M-X bond. We also obtained a strong linear relationship between E_X and the electron density ρ(r) at the bond critical point of M-Cl bond trans to the X in [M^IICl₃X]ⁿ⁻ and this allows us to express the BDE_X(Pd) and BDE_X(Pt) in terms of only the ρ(r) and D_X. We have demonstrated that using a database comprising of D_X and the ρ(r), the bond dissociation energy of X in complexes of the type [M^IIX(Y)X′(Y′)] can be predicted.     

Oxidative reactivity difference among the metal oxo and metal hydroxo moieties: pH dependent hydrogen abstraction by a manganese(IV) complex having two hydroxide ligands

Clarifying the difference in redox reactivity between the metal oxo and metal hydroxo moieties for the same redox active metal ion in identical structures and oxidation states, that is, M(n+)O and M(n+)-OH, contributes to the understanding of nature's choice between them (M(n+)O or M(n+)-OH) as key active intermediates in redox enzymes and electron transfer enzymes, and provides a basis for the design of synthetic oxidation catalysts. The newly synthesized manganese(IV) complex having two hydroxide ligands, [Mn(Me(2)EBC)(2)(OH)(2)](PF(6))(2), serves as the prototypic example to address this issue, by investigating the difference in the hydrogen abstracting abilities of the Mn(IV)O and Mn(IV)-OH functional groups. Independent thermodynamic evaluations of the O-H bond dissociation energies (BDE(OH)) for the corresponding reduction products, Mn(III)-OH and Mn(III)-OH(2), reveal very similar oxidizing power for Mn(IV)O and Mn(IV)-OH (83 vs 84.3 kcal/mol). Experimental tests showed that hydrogen abstraction proceeds at reasonable rates for substrates having BDE(CH) values less than 82 kcal/mol. That is, no detectable reaction occurred with diphenyl methane (BDE(CH) = 82 kcal/mol) for both manganese(IV) species. However, kinetic measurements for hydrogen abstraction showed that at pH 13.4, the dominant species Mn(Me(2)EBC)(2)(O)(2), having only Mn(IV)O groups, reacts more than 40 times faster than the Mn(IV)-OH unit in Mn(Me(2)EBC)(2)(OH)(2)(2+), the dominant reactant at pH 4.0. The activation parameters for hydrogen abstraction from 9,10-dihydroanthracene were determined for both manganese(IV) moieties: over the temperature range 288-318 K for Mn(IV)(OH)(2)(2+), DeltaH(double dagger) = 13.1 +/- 0.7 kcal/mol, and DeltaS(double dagger) = -35.0 +/- 2.2 cal K(-1) mol(-1); and the temperature range 288-308 K for for Mn(IV)(O)(2), DeltaH(double dagger) = 12.1 +/- 1.8 kcal/mol, and DeltaS(double dagger) = -30.3 +/- 5.9 cal K(-1) mol(-1).     

Synthesis and characterization of molybdenum oxo complexes of two tripodal ligands: reactivity studies of a functional model for molybdenum oxotransferases

Reaction of the tetradentate ligand N-(2-hydroxybenzyl)-N,N-bis(2-pyridylmethyl)amine (L-OH) with MoO2Cl2 in methanol in the presence of NaOMe and PF6- results in the formation of [MoO2(L-O)]PF6. Similarly, the reaction of N-(2-mercaptobenzyl)-N,N-bis(2-pyridylmethyl)amine (L-SH) with MoO2(acac)2 leads to the formation of [MoO2(L-S)]+. The dioxo-molybdenum complex [MoO2(L-O)]+ reacts with phosphines in methanol to afford phosphine oxides and an air-sensitive molybdenum complex, tentatively identified as [Mo(IV)O(L-O)(OCH3)]. The latter complex is capable of reducing biological oxygen donors such as DMSO or nitrate, thereby mimicking the activity of DMSO reductase and nitrate reductase. Reaction of [MoO2(L-O)]PF6 with PPh3 in other solvents than methanol leads to the formation of the Mo(V) dimer [(L-O)OMo(micro-O)MoO(L-O)](PF6)2. The crystal structures of [MoO2(L-O)]PF6 and the micro-oxo bridged dimer are presented.     

The reactivity of areneseleninato complexes. New pentacoordinate nickel(II) complexes with benzeneseleninato ions and ethylenediamine as ligands

Summary Five-coordinate bis(Se-benzeneseleninato)tris(ethylenediamine)nickel(II) complexes are obtained by reaction of the Ni(H₂O)₂(XC₆H₄SeO₂)₂ complexes (X = H,p-Cl,m-Cl,m-Br orp-Me) with ethylenediamine. All the diaquo complexes react with three moles of ethylenediamine to form Se-seleninato derivatives. The compounds are characterized on the basis of far i.r. and near i.r. spectroscopy, electronic spectra and magnetochemical investigations. The most attainable geometry is the square pyramidal, probably slightly distorted; tentative assignments for the electronic spectra are proposed. Conductivity data indicate that these new complexes are nonelectrolytes; both areneseleninato and ethylenediamine behave as monodentate ligands. The magnetic moments show that all the complexes are of high-spin type, the values lying within the ranges observed for other high-spin five-coordinate nickel(II) complexes.     

Two mononuclear molybdenum(VI) oxo complexes with tridentate hydrazone ligands: Synthesis and crystal structures

Reaction of [MoO₂(Acac)₂] (Acac = acetylacetonate) with two similar hydrazone ligands in methanol yielded two mononuclear molybdenum(VI) oxocomplexes with general formula [MoO₂(L)(CH₃OH)], where L = L¹ = (4-nitrophenoxy)acetic acid [1-(3-ethoxy-2-hydroxyphenyl)methylidene]hydrazide (H₂L¹) and L = L² = (4-nitrophenoxy)acetic acid [1-(5-bromo-2-hydroxyphenyl)methylidene]hydrazide (H₂L²). Crystal and molecular structures of the complexes were determined by single crystal X-ray diffraction method. All investigated compounds were further characterized by elemental analysis and FT-IR spectra. Single crystal X-ray structural studies indicate that the hydrazone ligands coordinate to the MoO₂ cores through enolate oxygen, phenolate oxygen, and azomethine nitrogen. The Mo atoms in both complexes are in octahedral coordination.     

Chromotropism studies on copper(II) compounds. Part II. Dinuclear copper(II) complexes with triply-bridged hydroxo,acetate, and halo ligands

Abstract

Two triply-bridged dinuclear copper(II) complexes of formula [LCu(μ-OH)(μ-OAc)(μ-X)CuL]X?0.5H₂O where L is a bidentate ligand of N-(pyridine-2-ylmethyl)propane-2-amine and X=Cl, 1 and Br, 2 were synthesized and characterized by elemental analyses, spectroscopic techniques (IR, UV–Vis, EPR), thermal analysis, conductance measurements, and single-crystal X-ray structure determination. The structures of both complexes are similar. The complexes show a distorted square-pyramidal arrangement around each copper(II) ion with a CuN₂O₂X chromophore in which both copper(II) ions are connected by a hydroxo bridge and a triatomic syn-syn carboxylato bridge in equatorial positions and a halide ion bridge at the axial site. The chromotropism behavior of the complexes, including solvato-, thermo-, and halochromism, were investigated in detail. Their halochromism was investigated in the pH range of 2.0–11.0 by visible absorption spectroscopy. The reversible color variations from blue to colorless are attributable to deprotonation and protonation of the ligands. The complexes show reversible thermochromism in solution due to dissociation and recombination of ligands to copper ions.     

Heteroligand complexes of chromium,manganese, and iron with trans-dibenzoporphyrazine and two oxo ligands: DFT calculations

Russian Chemical Bulletin - The possibility of formation of heteroligand (6,6,6,6)-macrotetracyclic chelate complexes of 3d-elements (M = Cr, Mn, Fe) with trans-dibenzoporphyrazine as the...     

Self-assembled monolayers of Ru/Os dinuclear complexes: probing monolayer structure and interaction energies by electrochemical means

Monolayers of [Ru(bpy)2(micro-1)M2][PF6]4 salts (M = Os, Ru; bpy = 2,2'-bipyridine, 1 = 4'-(2,2'-bipyridin-4-yl)-2,2':6',2' '-terpyridine, tpy = 2,2':6',2' '-terpyridine, and 2 = 4'-(4-pyridyl)-2,2':6',2' '-terpyridine) were self-assembled on platinum and investigated by fast-scan electrochemistry. The electrochemistry of the complexes in solution and confined to the surface in self-assembled monolayers (SAMs) exhibited an almost ideal behavior. Scan-rate-dependent measurements of the peak current density (jp) were used to determine interaction energies within the monolayer. It is shown that the tpy coordination sites of the dinuclear complexes interact more strongly within the SAM than the bipyridine-coordinated fragments. This result was supported by peak potential shifts, which are due to interaction forces in SAMs. The alignment of the rodlike complexes relative to the surface is discussed, and the results of molecular mechanics calculations indicate that the species adopt a tilted orientation.     

Dipole effects on cation-pi interactions: absolute bond dissociation energies of complexes of alkali metal cations to N-methylaniline and N,N-dimethylaniline

Threshold collision-induced dissociation of M (+)( nMA) x with Xe is studied using guided ion beam mass spectrometry, where nMA = N-methylaniline and N, N-dimethylaniline and x = 1 and 2. M (+) includes the following alkali metal cations: Li (+), Na (+), K (+), Rb (+), and Cs (+). In all cases, the primary dissociation pathway corresponds to the endothermic loss of an intact nMA ligand. The primary cross section thresholds are interpreted to yield 0 and 298 K bond dissociation energies (BDEs) for ( nMA) x-1 M (+)-( nMA) after accounting for the effects of multiple ion-neutral collisions, the internal and kinetic energy distributions of the reactants, and the dissociation lifetimes. Density functional theory calculations at the B3LYP/6-31G* level of theory are used to determine the structures of these complexes, which are also used in single-point calculations at the MP2(full)/6-311+G(2d,2p) level to determine theoretical BDEs. The results of these studies are compared to previous studies of the analogous M (+)(aniline) x complexes to examine the effects of methylation of the amino group on the binding interactions. Comparisons are also made to a wide variety of cation-pi complexes previously studied to elucidate the contributions that ion-dipole, ion-induced-dipole, and ion-quadrupole interactions make to the overall binding.     

Estimation of coordination bond energies of NH3, H2O and Et2NH ligands in the Ni(II) and Cu(II) complexes

Tridentate ligands 2-hydroxyphenylsalicylaldimine (SAPH₂), 2-hydroxyphenyl-2-hydroxy-1-naphtalaldimine (NAPH₂) and Ni(II) complexes with multidentate ligand Bis-N·N′-(salicylidene)-1,3-propanediamine (LH₂) as well as mononuclear complex of Cu(II) were prepared using the same multidentate ligand. Diethylamine (Et₂NH), NH₃ and H₂O monodentate ligands were bound to these complexes coordinatively. The heat absorbed at the temperatures where these ligands thermally dissociated from the complexes were measured using the TG and DSC methods. It is assumed that the states both of the complexes with and without the monodentate ligands are solid and coordination bond energy for the monodentate ligand is calculated. It is seen that these calculated coordination bond energies are comparable with hydrogen bond energies.     

Reactivity of rhenium(V) oxo Schiff base complexes with phosphine ligands: rearrangement and reduction reactions

The symmetric rhenium(V) oxo Schiff base complexes trans-[ReO(OH2)(acac2en)]Cl and trans-[ReOCl(acac2pn)], where acac2en and acac2pn are the tetradentate Schiff base ligands N,N'-ethylenebis(acetylacetone) diimine and N,N'-propylenebis(acetylacetone) diimine, respectively, were reacted with monodentate phosphine ligands to yield one of two unique cationic phosphine complexes depending on the ligand backbone length (en vs pn) and the identity of the phosphine ligand. Reduction of the Re(V) oxo core to Re(III) resulted on reaction of trans-[ReO(OH2)(acac2en)]Cl with triphenylphosphine or diethylphenylphosphine to yield a single reduced, disubstituted product of the general type trans-[Re(III)(PR3)2(acac2en)]+. Rather unexpectedly, a similar reaction with the stronger reducing agent triethylphosphine yielded the intramolecularly rearranged, asymmetric cis-[Re(V)O(PEt3)(acac2en)]+ complex. Reactions of trans-[Re(V)O(acac2pn)Cl] with the same phosphine ligands yielded only the rearranged asymmetric cis-[Re(V)O(PR3)(acac2pn)]+ complexes in quantitative yield. The compounds were characterized using standard spectroscopic methods, elemental analyses, cyclic voltammetry, and single-crystal X-ray diffraction. The crystallographic data for the structures reported are as follows: trans-[Re(III)(PPh3)2(acac2en)]PF6 (H48C48N2O2P2Re.PF6), 1, triclinic (P), a = 18.8261(12) A, b = 16.2517(10) A, c = 15.4556(10) A, alpha = 95.522(1) degrees , beta = 97.130(1) degrees , gamma = 91.350(1) degrees , V = 4667.4(5) A(3), Z = 4; trans-[Re(III)(PEt2Ph)2(acac2en)]PF6 (H48C32N2O2P2Re.PF6), 2, orthorhombic (Pccn), a = 10.4753(6) A, b =18.4315(10) A, c = 18.9245(11) A, V = 3653.9(4) A3, Z = 4; cis-[Re(V)O(PEt3)(acac2en)]PF6 (H33C18N2O3PRe.1.25PF6, 3, monoclinic (C2/c), a = 39.8194(15) A, b = 13.6187(5) A, c = 20.1777(8) A, beta = 107.7730(10) degrees , V = 10419.9(7) A3, Z = 16; cis-[Re(V)O(PPh3)(acac2pn)]PF6 (H35C31N2O3PRe.PF6), 4, triclinic (P), a = 10.3094(10) A, b =12.1196(12) A, c = 14.8146(15) A, alpha = 105.939(2) degrees , beta = 105.383(2) degrees , gamma = 93.525(2) degrees , V = 1698.0(3) A3, Z = 2; cis-[Re(V)O(PEt2Ph)(acac2pn)]PF6 (H35C23N2O3PRe.PF6), 5, monoclinic (P2(1)/n), a = 18.1183(18) A, b = 11.580(1) A, c = 28.519(3) A, beta = 101.861(2) degrees , V = 5855.9(10) A(3), Z = 4.     

One-dimensional oxalato-bridged Cu(II), Co(II), and Zn(II) complexes with purine and adenine as terminal ligands

The reaction of nucleobases (adenine or purine) with a metallic salt in the presence of potassium oxalate in an aqueous solution yields one-dimensional complexes of formulas [M(mu-ox)(H(2)O)(pur)](n) (pur = purine, ox = oxalato ligand (2-); M = Cu(II) [1], Co(II) [2], and Zn(II) [3]), [Co(mu-ox)(H(2)O)(pur)(0.76)(ade)(0.24)](n)(4) and ([M(mu-ox)(H(2)O)(ade)].2(ade).(H(2)O))(n) (ade = adenine; M = Co(II) [5] and Zn(II) [6]). Their X-ray single-crystal structures, variable-temperature magnetic measurements, thermal behavior, and FT-IR spectroscopy are reported. The complexes 1-4 crystallize in the monoclinic space group P2(1)/a (No. 14) with similar crystallographic parameters. The compounds 5 and 6 are also isomorphous but crystallize in the triclinic space group P (No. 2). All compounds contain one-dimensional chains in which cis-[M(H(2)O)(L)](2+) units are bridged by bis-bidentate oxalato ligands with M(.)M intrachain distances in the range 5.23-5.57 A. In all cases, the metal atoms are six-coordinated by four oxalato oxygen atoms, one water molecule, and one nitrogen atom from a terminal nucleobase, building distorted octahedral MO(4)O(w)N surroundings. The purine ligand is bound to the metal atom through the most basic imidazole N9 atom in 1-4, whereas in 5 and 6 the minor groove site N3 of the adenine nucleobase is the donor atom. The crystal packing of compounds 5 and 6 shows the presence of uncoordinated adenine and water crystallization molecules. The cohesiveness of the supramolecular 3D structure of the compounds is achieved by means of an extensive network of noncovalent interactions (hydrogen bonds and pi-pi stacking interactions). Variable-temperature magnetic susceptibility measurements of the Cu(II) and Co(II) complexes in the range 2-300 K show the occurrence of antiferromagnetic intrachain interactions.